Reflections at the interfaces between dissimilar signal propagation media used within measurement systems increase rapidly with frequency. These reflections are undesirable as they reduce the raw directivity of directional-coupler-based systems such as vector network analyzers (VNA) and other reflectometer-based instruments. A significant source of reflection (and thus reduced raw directivity) is the interface between a measurement system (i.e. VNA), which commonly includes semiconductor circuitry in planar or monolithic format, and a propagation medium connecting the measurement system to a load or device under test (DUT). Maintaining high raw directivity with increasing frequency can be difficult. For example, existing techniques for interfacing a reflectometer with a coaxial section exhibit standing waves across the interface that make this interface unusable at high frequencies (e.g., frequencies that exceed 70 GHz). There is thus a need for methods and devices for interfacing dissimilar propagation media in a low-reflection manner so as to maintain raw coupler directivity when propagating high frequency signals.